Process for the production of high purity melamine from urea

Abstract

The present invention concerns process for obtaining melamine at high purity characterized by comprising the steps of:—quenching a melamine melt, deprived of off-gases of the melamine synthesis and containing melamine impurities such as ammeline, ammelide and polycondensates, with an aqueous solution comprising ammonia under conditions such to substantially convert polycondensates into melamine, obtaining a melamine solution substantially free of polycondensates, recovering melamine from said melamine solution by melamine crystallization, obtaining melamine crystals and a melamine crystallization mother liquor,—treating said crystallization melamine mother liquor under conditions such to convert at least a portion of its ammeline content into ammelide and melammine, obtaining a treated mother liquid having at least a reduced ammeline content. The invention also concerns a high-pressure non-catalytic plant for the production of high-purity melamine implementing the above process and apparatuses suitable to be used in this plant.

The invention claimed is: 
     
       1. A process for obtaining melamine at high purity comprising the steps of:
 quenching a melamine melt, deprived of off-gases of the melamine synthesis and containing melamine impurities such as ammeline, ammelide and polycondensates, by mixing said melamine melt with an aqueous solution comprising ammonia under conditions such to substantially convert polycondensates into melamine, obtaining a melamine solution substantially free of polycondensates, 
 recovering melamine from said melamine solution by melamine crystallisation, obtaining melamine crystals and a melamine crystallization mother liquor, 
 thermally treating said crystallization melamine mother liquor by converting at least a portion of the ammeline contained in said melamine crystallization mother liquor into ammelide and melammine, obtaining a treated mother liquid having an ammeline content less than that of said melamine crystallization mother liquor. 
 
     
     
       2. The process according to  claim 1 , wherein said quenching step is carried out at a temperature in the range of 150° C. to 190° C. with an aqueous solution comprising ammonia at a concentration in the range from 20% to 35% for a time from 10 minutes to 30 minutes to obtain a melamine in which the melamine concentration is in the range from 8 to 16% by weight. 
     
     
       3. The process according to  claim 2 , wherein gaseous ammonia is also added to said aqueous solution comprising ammonia in the quenching step. 
     
     
       4. The process according to  claim 2 , wherein said quenching step is carried out at a temperature in the range from 160° C. to 180° C. and said melamine concentration in the melamine solution is in the range from 9% to 13% by weight. 
     
     
       5. The process according to  claim 1 , wherein said step of melamine recovering is carried out by cooling down said melamine solution to 30-60° C., preferably 40-50° C., so causing separation of melamine crystals. 
     
     
       6. The process according to  claim 1 , wherein said treating step comprises the operative stages of heating up said crystallization melamine mother liquor to a temperature in the range from 150° C. to 190° C. and keeping the heated mother liquor at that temperature for a time of 1 to 4 hours. 
     
     
       7. The process according to  claim 1 , wherein said aqueous solution containing ammonia comprises at least a portion of said treated mother liquid obtained in the treating step and recycled to the quenching step. 
     
     
       8. The process according to  claim 7 , further comprising the steps of:
 separating a portion of mother liquor from the treated mother liquor obtained in said treatment step, 
 substantially removing ammelide from said separated portion of mother liquor, and 
 returning said separated portion of mother liquor substantially deprived of ammelide to said treated mother liquid. 
 
     
     
       9. The process according to  claim 8 , wherein said portion of mother liquor separated from the treated mother liquor is comprised in the range of 5% to 20% of the whole treated mother liquid. 
     
     
       10. The process according to  claim 8 , wherein said step of removing ammelide from said separated portion of mother liquor comprises the operative stages of:
 stripping ammonia from said separated portion of mother liquor, so causing ammelide to precipitate, and 
 separating solid ammelide by filtration.